This invention relates to a pulse width modulated light projection system, and more particularly, to a technique for operating a pulse width modulated light projection system to minimize motion contouring.
Presently, there exists a type of semiconductor device, known as a Digital Micromirror Device (DMD), comprising a plurality of individually movable micromirrors arranged in a rectangular array. Each micromirror pivots about limited arc, typically on the order of 10-12xc2x0 under the control of a corresponding driver cell that latches a bit therein. Upon the application of a previously latched xe2x80x9c1xe2x80x9d bit, the driver cell causes its associated micromirror cell to pivot to a first position. Conversely, the application of a previously latched xe2x80x9c0xe2x80x9d bit to the driver cell causes the driver cell to pivot its associated micromirror to a second position. By appropriately positioning the DMD between a light source and a projection lens, each individual micromirror of the DMD device, when pivoted by its corresponding driver cell to the first position, will reflect light from the light source through the lens and onto a display screen to illuminate an individual picture element (pixel) in the display. When pivoted to its second position, each micromirror reflects light away from the display screen, causing the corresponding pixel to appear dark. An example of such DMD device is the DMD of the DLP(trademark) projection system available from Texas Instruments, Dallas Tex.
Present day projection systems that incorporate a DMD of the type described control the brightness (illumination) of the individual pixels by controlling the duty cycle during which the individual micromirrors remain xe2x80x9conxe2x80x9d (i.e., pivoted to their first position), versus the interval during which the micromirrors remain xe2x80x9coffxe2x80x9d (i.e. pivoted to their second position). To that end, such present day DMD-type projection systems use pulse width modulation to control the pixel brightness by varying the duty cycle of each micromirror in accordance with the state of the pulses in a sequence of pulse width segments. Each pulse width segment comprises a string of pulses of different time duration. The state of each pulse in a pulse width segment (i.e., whether each pulse is turned on or off) determines whether the micromirror remains on or off for the duration of that pulse. In other words, the larger the sum of the widths of the pulses in a pulse width segment that are turned on (actuated), the longer the duty cycle of each micromirror.
In a television projection system utilizing a DMD, the frame interval, i.e., the time between displaying successive images, depends on the selected television standard. The NTSC standard currently in use in the United States requires a frame interval of {fraction (1/60)} second whereas certain European television standards employ a frame interval of {fraction (1/50)} second. Present day DMD-type television projection systems typically achieve a color display by projecting red, green, and blue images either simultaneously or in sequence during each frame interval. A typical sequential DMD-type projection system utilizes a motor-driven color wheel interposed in the light path of the DMD. The color wheel has a plurality of separate primary color windows, typically red, green and blue, so that during successive intervals, red, green, and blue light, respectively, falls on the DMD. To achieve a color picture, red, green and blue light must fall on the DMD at least once within each successive frame interval. If only one red, one green and one blue image is made and each consumes ⅓ of the frame interval, then the large interval between colors will produce perceptible color breakup with motion. Present day DMD systems address this problem by breaking each color into several intervals and interleaving the intervals in time, thereby reducing the delay between colors.
Pulse width modulated projection systems of the type described above that have the ability to make multiple images of each primary color during each frame interval to yield a color picture often suffer from motion contouring on small amplitude transients, such as those associated with motion in a scene or motion of the viewer""s eyes. This type of artifact results from changes in the distribution of the light pulses across different portions of the display period.
U.S. Pat. No. 5,986,640 discloses a scheme for reducing motion contouring by splitting the most significant bits in a sequence of pulse width segments between two or more time-adjacent segments (intervals). While this scheme serves to reduce contouring, it does not eliminate contouring on all transitions. Further, splitting bits in a manner sufficient to reduce contouring will increase the number of times each pixel must be addressed, thereby increasing the bandwidth needed to accomplish such addressing.
Thus, there is a need for a technique for operating a pulse width modulated display to reduce the motion contouring while overcoming the aforementioned disadvantages of the prior art.
In accordance with present principles, there is provided a method for operating a pulse width modulated display system, such as a pulse width modulated display system that incorporates a Digital Micromirror Device (DMD), to selectively reflect light from a light source through a projection lens and onto a display screen. In such a display system, the illumination of each pixel for a given color is controlled responsive to pulses within a sequence of pulse width segments. The state of each pulse in each segment determines whether the pixel becomes illuminated during the interval associated with that pulse. To reduce the incidence of motion contouring, pixel brightness is increased by actuating selected pulses such that within a first range of brightness levels between first and second pixel brightness boundaries, a first large-duration pulse (or combination of pulses) becomes actuated to reach the second pixel brightness boundary. Within a second range of pixel brightness levels between second and third pixel brightness boundaries, the first large duration pulse (or combination of pulses) remains actuated, and upon reaching the third pixel brightness boundary, a second large duration pulse (or combination of pulses) also becomes actuated, with the first large duration pulse remaining actuated.
As the pixel brightness increases, another yet un-actuated large duration pulse (or combination of pulses) becomes actuated upon reaching a successively higher pixel brightness boundary, with each already actuated large duration pulse (or combination of pulses) remaining actuated. Each large duration pulse (or combination of pulses) that becomes actuated at each pixel brightness boundary is referred to as a xe2x80x9cthermometer codexe2x80x9d pulse because once actuated, that pulse (or combination of pulses) remains actuated upon further increases in pixel brightness above that brightness boundary in a manner analogous to a temperature level on a mercury thermometer. Depending on the width (i.e., duration) of each of the pulses within each segment, a given segment can include more than one such thermometer code pulse. However, upon an increase in pixel brightness to reach a given pixel brightness boundary, only a single previously de-actuated thermometer code pulse changes state (i.e., becomes actuated). Conversely, when the pixel brightness is decreased to a given pixel brightness boundary, only a single thermometer code pulse that had been actuated becomes now de-actuated, with the other thermometer code pulses that have yet to be de-actuated thus remaining actuated.